Building block



FIG. 1 is a top front right perspective view of a building block in accordance with my new design;

FIG. 2 is a bottom front left perspective view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom view thereof;

FIG. 5 is a right elevation view thereof; and,

FIG. 6 is a front elevation view thereof. 

CLAIM The ornamental design for a building block, as shown and described. 